CeACAM3 (CD66d), a member of carcinoembryonic antigen family, is a granulocyte-specific receptor involved in the opsonin-independent recognition of several bacterial pathogens. There are four members in this family: CD66a, CD66b, CD66c, and CD66d. Members of CEACAM family are widely expressed especially on human neutrophils, and, depending on the tissue, capable of regulating diverse functions including tumor promotion, tumor suppression, angiogenesis, and neutrophil activation. Abnormal overexpression and downregulation of some CEACAMs have been described in tumor cells. Monoclonal antibodies grouped in the CD66 cluster recognize CEACAM members. Ectopic CD66 expression is commonly detected in B-cell lineage acute lymphoblastic leukemia (ALL). CEACAM3 mediates phagocytosis depends on the integrity of an ITAM-like sequence within the cytoplasmic domain of CEACAM3. CEACAM3 is characterized by rapid stimulation of the GTPase Rac.